1. Field of the Invention
The present invention relates to a method for improving the signal-to-noise ratio of a signal and, more particularly, to such a method for removing acoustic reverberations from the received signal.
2. Setting of the Invention
When an acoustic transducer has an electrical current passing across it, the transducer material will expand or contract to generate a positive or negative respectively pulse of acoustic energy. The acoustic energy passes through the surrounding environment and portions of the acoustic energy will be reflected back to the transducer upon contacting boundaries between disparate mediums. A problem exists where the acoustic energy contacts a relatively thin boundary layer between two disparate mediums, because of the `ringing` caused by closely spaced reverberations within the layer. The magnitude and time duration of the ringing is such that reflections from other outer boundaries can be totally obscured, that is, the ratio of the signal desired to be investigated to the ringing from the thin layer is considered so poor as rendering an analysis of the signal ineffective. An example of this problem will be described below.
In the art of oil and gas production from a wellbore, ultrasonic transducers in contact with the well casing can be used to investigate the quality of the cement bond between the column of cement surrounding a casing set within a wellbore. This tool utilizes one or more acoustic energy transducers to transmit acoustic energy pulses that pass through the casing, cement, and the surrounding formation. The acoustic reflections are received within the tool to produce a signal for analysis by an operator at the surface. The acoustic problem exists here because a thin, highly acoustically mismatched medium, i.e., the casing, exists between the transducer and the medium to be interrogated, i.e., the cement. As stated above, the casing causes ringing, and this ringing obscures the portions of the received signals of interest, such as reflections from cement defects interface and the cement-formation interface.
Various commonly known numerical methods can be used to improve the signal-to-noise ratio, such as those used by those skilled in the art in geophysics. These methods require the use of digital computers and cannot be practically implemented in a downhole well logging tool.